It is relatively common practice to steer an off-the-road vehicle by independently controlling right and left driven wheels whereby the steering is achieved by driving one set of right or left wheels at a differing speed or direction with respect to the other set of wheels. Thus, steering can be achieved in a relatively simple manner, and the mechanism used for steering is also used for driving the vehicle at varying speeds in forward and reverse directions. In a typical example, a vehicle is provided with right and left drive wheels which are driven by independent hydraulic motors. The right hydraulic motor is in turn connected to a right variable displacement pump, and the left hydraulic motor is connected to a left variable displacement pump. Each of the variable displacement pumps is provided with a control arm which varies the position of a swash plate within the pump. By varying the position of the control arms the output of the pumps are varied causing the motors to be driven at varying speeds and directions. Thus, the speed of the vehicle can be varied and the vehicle can also be turned by causing one motor to rotate faster than the other or even by causing one motor to rotate in a differing direction than the other.
A typical representation of the prior art is U.S. Pat. No. 3,897,840. It can be seen from an inspection of this patent that the vehicle speed and direction control system differs from the normal over the highway control system. Thus, in this patent, a steering wheel is provided which facilitates the steering of the vehicle, but the speed in both forward and reverse directions is controlled by a hand lever. Thus, as the control system is somewhat different from that normally encountered by a highway vehicle, it is necessary to provide the operator either with prior instruction and/or time to familiarize himself with the vehicle operation.
U.S. Pat. No. 3,550,708 also discloses a similar system wherein both a steering wheel and a foot pedal control are provided. However, in this design turning is directly proportional to the speed of the vehicle. Thus, at a stop position, no steering can be achieved whereas, in the prior patent, a pivot steer can be achieved. At a slow speed, only a slow rate of steer can be achieved, however, at high speed, a high steering rate can be achieved, which is of course, undesirable.